What dose "brain-melting" mean?

In the late '60s, New York's equivalent of Haight-Ashbury was Greenwich Village and various contiguous neighborhoods extending to the Lower East Side. It's where the head shops were – their walls and ceilings surrealistic galleries plastered with the latest eye-popping, brain-melting rock posters and black light creations; their air thick with sandlewood incense.

1. What dose "brain-melting" mean in the paragraph?

2. "Black light" means just "Dark" or does it have other meaning?

Thanks a lot for your kind help.

Do Hoon.

Dear Friends,

(Black light) is : An item that is "black light sensitive" will fluoresce, or glow when exposed to ultraviolet light.

I wish you all the best in your live.

Yours Sincerly

Diplomatic